The MAY-SEP 2023 U.S. population weight CDD forecast is adjusted. The outlook trends slightly warmer. Note that the outlook is consistently warmer than the 30-year climatology but not as hot as the majority of the last 3 years through July followed by AUG/SEP forecasts which are similar.

An area of low pressure is forming off the East Coast of Georgia along a quasi-stationary frontal system. Plumes of thunderstorm activity are ahead of this low-pressure area. Near the low-pressure area, a plume of water ocean surface water associated with Gulf Stream is in the 27C range.

The chart above identifies the warming of the atmosphere by a warm water zone known as a semi-permanent marine heat wave causing a semi-permanent upper-level high-pressure ridge (red) known as the ridiculously resilient ridge. The location of the ridge for each of the past 9 years is identified.

Presence of an upper trough across South-central Europe much of the next 2 weeks entraining buoyant low-level atmosphere moisture from the much warmer than normal western Mediterranean Sea causes a large area of heavy rain in the latest GFS 15-day forecast.